Rubber composition for tires

Abstract

To provide a rubber composition for tires that can meet high standards for both on-ice performance and wear resistance.SOLUTION: A rubber composition for tires includes 100 pts.mass of a diene rubber, blended with 30 pts.mass or more of a white filler, and 0.5-30 pts.mass of particles with a glass transition temperature of -70°C to 0°C. The particles used here include a polymer of a monomer mixture including 80 mass%-99.5 mass% of a monofunctional (meth)acrylate monomer, 0.1 mass%-5 mass% of a diene monomer, and 0.4 mass%-15 mass% of a polyfunctional vinyl monomer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates primarily to a rubber composition for tires intended for use in the tread portion of studless tires. [Background technology]

[0002] It is known that increasing the surface roughness (undulation) of the tread rubber is a way to improve the ice performance of studless tires. It is thought that increasing the surface roughness of the tread rubber has the effect of increasing the contact area with the ice surface compared to tread rubber with a smooth surface, as the recessed areas trap the water film present on the ice surface and the raised areas come into contact with the ice surface.

[0003] For example, Patent Document 1 proposes incorporating resin particles (acrylic resin) having specific properties into the rubber composition that constitutes the tread rubber. In this proposal, the incorporation of resin particles creates fine irregularities on the surface of the tread rubber, which can improve ice performance. However, simply incorporating resin particles may result in the resin particles falling off the tread rubber during driving, potentially failing to achieve the desired ice performance or ensuring abrasion resistance. Therefore, when incorporating resin particles into a rubber composition to ensure ice performance, measures are needed to ensure abrasion resistance while exhibiting excellent ice performance. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2018-123209 [Overview of the project] [Problems that the invention aims to solve]

[0005] The object of the present invention is to provide a rubber composition for tires that enables a high degree of balance between ice performance and wear resistance. [Means for solving the problem]

[0006] The present invention provides a rubber composition for tires that achieves the above objective, comprising 100 parts by mass of diene rubber, 30 parts by mass or more of a white filler, and 0.5 to 30 parts by mass of particles having a glass transition temperature of -70°C to 0°C, wherein the particles consist of a polymer of a monomer mixture containing monomer (A), monomer (B), and monomer (C), wherein monomer (A) is a monofunctional (meth)acrylic acid ester monomer, and monomer (B) is It has at least two (meth)acryloyl groups, a weight-average molecular weight of 500 to 50000, and is represented by the following general formula (1). It is a diene monomer, R 1 -OR 2 -OR 3 (1) (In the formula, R 1 and R 3 is a (meth)acryloyl group, R 2 This structure includes polymer chains that have dienes as constituent units. The monomer (C) is a polyfunctional vinyl monomer other than monomer (B), and is characterized in that the mass ratio of monomer (A) in the monomer mixture is 80% to 99.5% by mass, the mass ratio of monomer (B) in the monomer mixture is 0.1% to 5% by mass, and the mass ratio of monomer (C) in the monomer mixture is 0.4% to 15% by mass. [Effects of the Invention]

[0007] The rubber composition for studless tires of the present invention consists of polymers of the above-mentioned monomer mixture, and contains particles with a glass transition temperature of -70°C to 0°C, thereby improving ice performance and wear resistance to levels beyond conventional levels. In particular, because these particles consist of polymers of a monomer mixture containing monofunctional (meth)acrylic acid ester monomers and polyfunctional vinyl monomers, the flexibility and resistance to crushing of the particles can be ensured, and it is believed that they can effectively exhibit ice performance when used in tires. Furthermore, since the monomer mixture contains not only monofunctional (meth)acrylic acid ester monomers and polyfunctional vinyl monomers but also diene monomers, the resulting particles have good affinity for the diene rubber, which is the main component of the rubber composition, and the particles are less likely to fall out of the rubber composition, thus reliably exhibiting ice performance and improving wear resistance. The glass transition temperature is measured by differential scanning calorimetry (DSC) under a heating rate of 20°C / min, and the temperature at the midpoint of the transition region is taken.

[0008] In the rubber composition for studless tires of the present invention, the diene rubber preferably contains butadiene rubber and natural rubber, with a natural rubber content of 30 parts by mass or more per 100 parts by mass of butadiene rubber. Including a sufficient amount of natural rubber relative to the butadiene rubber in this way is advantageous for achieving both ice performance and wear resistance.

[0009] In the rubber composition for studless tires of the present invention, monomer (B) preferably has at least two (meth)acryloyl groups, a weight-average molecular weight of 500 to 50000, and is represented by the following general formula (1). Compounds represented by the following general formula (1) have a reactive carbon-carbon double bond, such as a polybutadiene skeleton, in their molecules, which is advantageous for increasing affinity to diene-based rubbers, which are the main components of the rubber composition. On the other hand, by having the more reactive functional group, the (meth)acryloyl group, the aforementioned reactive carbon-carbon double bond is not used in the polymerization reaction during particle production, thus effectively increasing affinity to diene-based rubbers in the rubber composition. R 1 -O-R 2 -O-R 3 (1) (In the formula, R 1 and R 3 are (meth)acryloyl groups, and R 2 has a structure containing a polymer chain having a diene as a structural unit.)

[0010] In the studless tire rubber composition of the present invention, it is preferable that the average particle diameter of the particles is 3 μm to 70 μm. This is advantageous for achieving both ice performance and wear resistance performance.

[0011] In the studless tire rubber composition of the present invention, it is preferable that the glass transition temperature is -60°C or lower. This is advantageous for achieving both ice performance and wear resistance performance.

[0012] The studless tire rubber composition of the present invention preferably further contains a liquid polymer having a weight average molecular weight of 1000 to 100000. Further, the studless tire rubber composition of the present invention preferably further contains a terminally modified diene rubber. Furthermore, the studless tire rubber composition of the present invention preferably further contains an aromatic modified terpene resin. By making such a formulation, it is advantageous for achieving both ice performance and wear resistance performance.

[0013] The studless tire rubber composition of the present invention can be used for the tread portion of a studless tire. A studless tire formed with the tread portion using the studless tire rubber composition of the present invention can effectively exhibit ice performance and wear resistance performance due to the physical properties of the above-mentioned studless tire rubber composition.

Embodiments for Carrying Out the Invention

[0014] In the tire rubber composition of the present invention, the rubber component is a diene-based rubber, and for example, natural rubber, isoprene rubber, butadiene rubber, styrene-butadiene rubber, styrene-isoprene rubber, styrene-isoprene-butadiene rubber, acrylonitrile-butadiene rubber, etc. can be used. Among these, natural rubber, butadiene rubber, and styrene-butadiene rubber are preferred, and natural rubber and butadiene rubber are particularly preferred.

[0015] When using natural rubber and butadiene rubber, it is preferable that the total amount of butadiene rubber in 100 parts by mass of natural rubber and butadiene rubber contains preferably 20% to 80% by mass, more preferably 30% to 70% by mass. Alternatively, it is preferable that the total amount of natural rubber in 100 parts by mass of natural rubber and butadiene rubber contains preferably 20% to 80% by mass, more preferably 30% to 70% by mass. In this case, the content of natural rubber is preferably 30% or more by mass, more preferably 60% to 300% by mass, relative to 100% by mass of butadiene rubber. By composing the rubber components with natural rubber and butadiene rubber in this way and incorporating them in a predetermined blend, both ice performance and abrasion resistance can be achieved. If the content of natural rubber relative to 100 parts by mass of butadiene rubber is less than 30% by mass, abrasion resistance will decrease.

[0016] In the tire rubber composition of the present invention, the diene rubber may include a terminally modified diene rubber modified with an organic compound having a functional group at one or both ends of the molecular chain. Examples of terminally modified diene rubbers include terminally modified butadiene rubber or terminally modified styrene-butadiene rubber. Examples of functional groups that modify the ends of the molecular chain include alkoxysilyl groups, hydroxyl groups, aldehyde groups, carboxyl groups, amino groups, amide groups, imino groups, alkoxyl groups, epoxy groups, amide groups, thiol groups, ether groups, and siloxane bonding groups. Among these, siloxane, alkoxysilyl, and hydroxyl groups can be preferably used. The siloxane bonding group mentioned above is a functional group having an -O-Si-O- structure.

[0017] The average glass transition temperature of the diene-based rubber described above is preferably -50°C or lower, and more preferably -100°C to -60°C. By setting the average glass transition temperature of the diene-based rubber to -50°C or lower, the flexibility of the rubber compound at low temperatures is maintained and the adhesion to ice surfaces is increased, making it suitable for use in the tread portion of studless tires. The glass transition temperature is measured by differential scanning calorimetry (DSC) under a heating rate of 20°C / min, and the temperature at the midpoint of the transition region is taken. If the diene-based rubber is an oil-expanded product, the glass transition temperature of the diene-based rubber in a state without oil-expanded components is taken. The average glass transition temperature is the sum of the glass transition temperatures of each diene-based rubber multiplied by the mass fraction of each diene-based rubber (weighted average value of glass transition temperatures). The sum of the mass fractions of all diene-based rubbers is set to 1.

[0018] The rubber composition for studless tires of the present invention must contain a white filler. Examples of white fillers include silica, calcium carbonate, magnesium carbonate, talc, clay, alumina, aluminum hydroxide, titanium dioxide, and calcium sulfate. These may be used individually or in combination of two or more. Among these, silica is preferred as it can improve ice performance. Examples of silica include wet silica (hydrated silica), dry silica (anhydrous silica), calcium silicate, and aluminum silicate, and these may be used individually or in combination of two or more.

[0019] The amount of white filler added is 30 parts by mass or more, preferably 35 to 80 parts by mass, per 100 parts by mass of diene rubber. Adding 30 parts by mass or more of white filler improves the mechanical properties of the rubber composition and enhances wear resistance. Adding 35 to 80 parts by mass of white filler not only improves the mechanical properties of the rubber composition and enhances wear resistance, but also maintains the flexibility of the rubber composition and effectively ensures ice performance. If the amount of white filler added is less than 30 parts by mass, it becomes difficult to ensure sufficient mechanical properties of the rubber composition.

[0020] The specific surface area of ​​silica adsorbing CTAB is not particularly limited, but is preferably 80 m². 2 / g~260m 2 / g, more preferably 140m 2 / g~200m 2 It is preferable that the specific surface area of ​​silica adsorbing CTAB is 80 m². 2 By increasing the amount to over / g, the abrasion resistance of the rubber composition can be ensured. Furthermore, the specific surface area of ​​silica adsorbed with CTAB can be increased to 200 m². 2 By reducing the amount to less than / g, wet performance and low rolling resistance can be improved. In this specification, the CTAB specific surface area of ​​silica shall be the value measured according to ISO 5794.

[0021] In this invention, it is preferable to incorporate a silane coupling agent together with silica. By incorporating a silane coupling agent, the dispersibility of silica in diene-based rubber can be improved, and a better balance between wear resistance and ice performance can be achieved.

[0022] The type of silane coupling agent is not particularly limited as long as it can be used in silica-containing rubber compositions, but examples of sulfur-containing silane coupling agents include bis-(3-triethoxysilylpropyl)tetrasulfide, bis(3-triethoxysilylpropyl)disulfide, 3-trimethoxysilylpropylbenzothiazoletetrasulfide, γ-mercaptopropyltriethoxysilane, and 3-octanoylthiopropyltriethoxysilane.

[0023] When incorporating a silane coupling agent, the amount should preferably be 3% to 15% by mass, more preferably 5% to 10% by mass, relative to the mass of silica. If the amount of silane coupling agent is less than 3% by mass of silica, the dispersion of silica cannot be sufficiently improved. If the amount of silane coupling agent exceeds 15% by mass of silica, the silane coupling agents will condense with each other, making it impossible to obtain the desired hardness and strength in the rubber composition.

[0024] The tire rubber composition of the present invention may further contain carbon black in addition to the white filler described above. Examples of carbon black include furnace carbon blacks such as SAF, ISAF, HAF, FEF, GPF, HMF, and SRF, which may be used individually or in combination of two or more. The amount of carbon black added is preferably 3 to 70 parts by mass, more preferably 5 to 60 parts by mass, per 100 parts by mass of diene rubber. Adding 3 parts by mass or more of carbon black can improve the mechanical properties of the rubber composition and enhance wear resistance. Adding 70 parts by mass or less of carbon black can maintain the flexibility of the rubber composition and ensure ice performance. It can also suppress an increase in mass when made into a tire.

[0025] When carbon black is also included in addition to the white filler, the nitrogen adsorption specific surface area of ​​the carbon black is preferably 70 m². 2 / g~240m 2 / g, more comfortably 90m 2 / g~200m 2 It is desirable that the nitrogen adsorption specific surface area of ​​carbon black be 70 m². 2 By increasing the nitrogen adsorption specific surface area of ​​carbon black to 240 m² or more, the mechanical properties and wear resistance of the rubber composition can be ensured. 2 By reducing the nitrogen adsorption specific surface area to less than / g, ice performance can be improved. In this specification, the nitrogen adsorption specific surface area of ​​carbon black shall be measured in accordance with JIS K6217-2.

[0026] The rubber composition for tires of the present invention preferably contains a liquid polymer. By incorporating a liquid polymer, the rubber hardness at low temperatures can be made more flexible, resulting in improved ice performance. Examples of liquid polymers include liquid polybutene, liquid polyisobutene, liquid polyisoprene, liquid polybutadiene, liquid poly-α-olefin, liquid isobutylene, liquid ethylene-α-olefin copolymer, liquid ethylene propylene copolymer, and liquid ethylene-butylene copolymer. The liquid polymer may also be various modified versions of the above-mentioned liquid polymers (maleic acid modification, terminal isocyanate modification, epoxy modification, etc.). The liquid polymer used in the present invention is liquid at room temperature (23°C). Therefore, it is distinguished from the aforementioned diene-based rubbers, which are solid at this temperature.

[0027] When incorporating a liquid polymer, it is preferable to use a liquid polymer with a weight-average molecular weight of preferably 1,000 to 100,000, more preferably 2,000 to 90,000. If the weight-average molecular weight of the liquid polymer is less than 1,000, sufficient ice performance after aging cannot be obtained. If the weight-average molecular weight of the liquid polymer exceeds 100,000, sufficient improvement in ice performance cannot be obtained. The amount of liquid polymer to be incorporated is not particularly limited, but it is preferably 3 to 80 parts by mass, more preferably 5 to 60 parts by mass, per 100 parts by mass of rubber component. If the amount of liquid polymer is less than 3 parts by mass, sufficient improvement in ice and snow performance cannot be obtained. If the amount of liquid polymer exceeds 80 parts by mass, there is a risk that the tensile breaking strength will deteriorate. Note that the weight-average molecular weight refers to the weight-average molecular weight in polystyrene terms, as analyzed by gel permeation chromatography (GPC).

[0028] The rubber composition for tires of the present invention preferably contains an aromatically modified terpene resin. By incorporating an aromatically modified terpene resin, wet performance can be further improved. This is thought to be because the aromatically modified terpene resin improves the dispersibility of fillers such as silica and carbon black, and also improves the compatibility between the fillers and the rubber components. Examples of aromatically modified terpene resins include those obtained by polymerizing terpenes such as α-pinene, β-pinene, dipentene, and limonene with at least one aromatic compound selected from styrene, α-methylstyrene, and vinyltoluene.

[0029] As the aromatic modified terpene resin, an aromatic modified terpene resin having a softening point preferably between 60°C and 150°C, and more preferably between 80°C and 130°C, can be suitably used. If the softening point of the aromatic modified terpene resin is below 60°C, the effect of improving wet performance cannot be sufficiently obtained. If the softening point of the aromatic modified terpene resin exceeds 150°C, there is a concern that it may not dissolve completely during mixing. The softening point of the aromatic modified terpene resin shall be measured in accordance with JIS K6220-1 (ring-sphere method). The amount of aromatic modified terpene resin to be blended is not particularly limited, but it is preferably between 2 and 40 parts by mass, more preferably between 3 and 35 parts by mass, per 100 parts by mass of rubber component. If the amount of aromatic modified terpene resin blended is less than 2 parts by mass, the effect of improving wet performance cannot be sufficiently obtained. If the amount of aromatic modified terpene resin blended exceeds 40 parts by mass, there is a risk that the ice and snow performance will deteriorate.

[0030] The rubber composition for tires of the present invention improves ice performance and wear resistance by incorporating the particles described below. Particles having specific properties can be prepared, for example, by polymerizing a non-crosslinkable monomer in the presence of a crosslinkable monomer by suspension polymerization, seed polymerization, or dispersion polymerization. Furthermore, it is preferable that the particles described below are particles that are incompatible with diene-based rubber. Here, "incompatible with diene-based rubber" does not mean incompatible with all types of rubber components contained in diene-based rubber, but rather incompatible with the specific diene-based rubber contained in the tire rubber composition. The specific particles that are incompatible with diene-based rubber can improve ice performance by forming a phase separation structure with the diene-based rubber.

[0031] The resin constituting the particles used in the present invention consists of a polymer of a monomer mixture containing monomer (A), monomer (B), and monomer (C) as described below.

[0032] In the present invention, monomer (A) is a monofunctional (meth)acrylic acid ester monomer. A monofunctional (meth)acrylic acid ester monomer means a (meth)acrylic acid ester having one ethylenically unsaturated group in one molecule. Examples of monofunctional (meth)acrylic acid ester monomers include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, and lauryl (meth)acrylate; alicyclic alcohol (meth)acrylic acid esters such as cyclohexyl (meth)acrylate and isobornyl (meth)acrylate; aryl (meth)acrylates such as phenyl (meth)acrylate; and alkoxyalkyl (meth)acrylates such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)acrylate. These monofunctional (meth)acrylic acid ester monomers may be used individually or in combination of two or more. Among these, alkyl (meth)acrylic acid esters are preferred, alkyl acrylates are more preferred, and n-butyl acrylate, isobutyl acrylate, and 2-ethylhexyl acrylate are particularly preferred. (Meth)acrylic means acrylic and / or methacrylic.

[0033] Monomer (A), i.e., monofunctional (meth)acrylic acid ester monomer, is used in the monomer mixture in a proportion of 80% to 99.5% by mass. The inclusion of monomer (A) ensures the flexibility and resistance to crushing of the particles, allowing for effective ice performance when used in tires. If the mass proportion of monofunctional (meth)acrylic acid ester monomer in the monomer mixture is less than 80% by mass, when particles are added to the rubber composition, it is not possible to eliminate the sticky feeling on the surface while maintaining a soft feel. If the mass proportion of monofunctional (meth)acrylic acid ester monomer in the monomer mixture exceeds 99.5% by mass, the particles aggregate during drying, resulting in poor handling. The mass proportion of monomer (A) in the monomer mixture is preferably 85% to 99% by mass, more preferably 92.5% to 98% by mass.

[0034] In the present invention, monomer (B) is a diene monomer. As the diene monomer, those having at least two (meth)acryloyl groups, a weight-average molecular weight of 500 to 50000, and represented by the following general formula (1) are particularly suitable. The compound represented by the following general formula (1) has a reactive carbon-carbon double bond, such as a polybutadiene skeleton, in its molecule, which is advantageous for increasing its affinity for the diene rubber, which is the main component of the rubber composition. On the other hand, by having the more reactive functional group, the (meth)acryloyl group, the aforementioned reactive carbon-carbon double bond is not used in the polymerization reaction during particle production, thus effectively increasing the affinity for the diene rubber in the rubber composition. R 1 -OR 2 -OR 3 (1) (In the formula, R 1 and R 3 is a (meth)acryloyl group, R 2 This structure includes polymer chains that have dienes as constituent units.

[0035] In the diene monomer represented by general formula (1), if there is one or fewer (meth)acryloyl groups, the particle strength decreases. In the diene monomer represented by general formula (1), the affinity with diene rubber with a weight-average molecular weight of less than 500 cannot be improved. In the diene monomer represented by general formula (1), if the weight-average molecular weight exceeds 50,000, the reactivity of the diene monomer decreases. Note that the weight-average molecular weight refers to the weight-average molecular weight on a polystyrene basis as analyzed by gel permeation chromatography (GPC). Also, "(meth)acryloyl" means acryloyl and / or methacryloyl. In the diene monomer represented by general formula (1), the weight-average molecular weight is more preferably 600 to 35,000, even more preferably 1,000 to 30,000, and particularly preferably 1,500 to 25,000.

[0036] In the diene monomer represented by general formula (1), R in the formula 2 Examples of dienes that are constituent units of the polymer chains contained in include conjugated dienes such as butadiene, isoprene, and chloroprene; and unconjugated dienes such as 1,4-hexadiene and 5-ethylidene-2-norbornene. These dienes may be used individually or in combination of two or more. Among these, conjugated dienes are preferred, and butadiene is particularly preferred. Examples of diene monomers represented by general formula (1) include polybutadiene di(meth)acrylate and di(meth)acrylate having polyisoprene as the main chain skeleton. These diene monomers may be used individually or in combination of two or more. Among these, polybutadiene di(meth)acrylate is preferred. Note that "(meth)acrylate" means acrylate and / or methacrylate.

[0037] Monomer (B), i.e., diene monomer, is used in a proportion of 0.1% to 5% by mass in the monomer mixture. Because of the inclusion of diene monomer, the particles have good affinity for the diene rubber, which is the main component of the rubber composition. This prevents the particles from easily falling out of the rubber composition, ensuring reliable ice performance and improving abrasion resistance. If the mass proportion of diene monomer in the monomer mixture is less than 0.1% by mass, the affinity for the diene rubber cannot be improved. If the mass proportion of diene monomer in the monomer mixture exceeds 5% by mass, the flexibility of the particles deteriorates. The mass proportion of diene monomer in the monomer mixture is preferably 0.3% to 3% by mass, more preferably 0.5% to 1.5% by mass.

[0038] In the present invention, monomer (C) is a polyfunctional vinyl monomer other than monomer (B) described above. A polyfunctional vinyl monomer means a monomer (crosslinking agent) having at least two ethylenically unsaturated groups in one molecule. Examples of polyfunctional vinyl monomers include aromatic divinyl monomers such as divinylbenzene and divinylnaphthalene; and bifunctional or more (meth)acrylate monomers such as allyl methacrylate, triacrylic formal, ethylene glycol di(meth)acrylate, diethylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,9-nonanediol di(meth)acrylate, PEG#200 di(meth)acrylate, PEG#400 di(meth)acrylate, PEG#600 di(meth)acrylate, trimethylolpropane tri(meth)acrylate, pentaerythritol tri(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and 2-butyl-2-ethyl 1,3-propanediol di(meth)acrylate. These polyfunctional vinyl monomers may be used individually or in combination of two or more. Among these, trimethylolpropane trimethacrylate and ethylene glycol dimethacrylate are preferred, with ethylene glycol dimethacrylate being particularly preferred.

[0039] Monomer (C), i.e., polyfunctional vinyl monomer, is used in the monomer mixture in a proportion of 0.4% to 15% by mass. If the mass proportion of polyfunctional vinyl monomer in the monomer mixture is less than 0.4% by mass, the particles tend to aggregate during drying due to the low degree of crosslinking, resulting in poor handling. If the mass proportion of polyfunctional vinyl monomer in the monomer mixture exceeds 15% by mass, the particles become hard, and the soft feel is impaired when added to the rubber composition. The mass proportion of polyfunctional vinyl monomer in the monomer mixture is preferably 0.7% to 12% by mass, more preferably 1% to 7.5% by mass.

[0040] The particles used in this invention preferably have an average particle diameter of 3 μm to 70 μm, more preferably 5 μm to 60 μm. When the average particle diameter is 3 μm or more, the surface of the tread rubber can be deformed to correspond to minute irregularities on the ice surface, improving ice performance. When the average particle diameter exceeds 70 μm, the particles tend to fall off the surface of the tread rubber, which may worsen wear resistance. In this specification, the average particle diameter is the average value obtained by measuring the particle diameter of at least 100 particles from a microscope image taken at 1000x to 5000x magnification. When the particle image is not circular, the equivalent diameter of the circle obtained from its projected area can be used as the particle diameter.

[0041] The glass transition temperature of the particles used in this invention is -70°C to 0°C, preferably -65°C to -10°C, and more preferably -60°C to -15°C. By setting the glass transition temperature of the particles to 0°C or lower, the flexibility of the rubber compound at low temperatures can be maintained, the adhesion to the ice surface can be increased, and the particles can be suitably used in the tread portion of studless tires. If the glass transition temperature of the particles is below -70°C, the handling stability deteriorates. If the glass transition temperature of the particles exceeds 0°C, the ice performance deteriorates. The glass transition temperature is determined by measuring a thermogram using differential scanning calorimetry (DSC) at a heating rate of 20°C / min, and the temperature at the midpoint of the transition region is taken.

[0042] The rubber composition for tires of the present invention may contain various compounding agents commonly used in rubber composition for tires, such as vulcanizing or crosslinking agents, vulcanization accelerators, antioxidants, plasticizers, processing aids, and thermosetting resins. Such compounding agents can be kneaded in a conventional manner to form a rubber composition, which can then be used for vulcanization or crosslinking. The amounts of these compounding agents can be conventional amounts, as long as they do not contradict the purpose of the present invention. The rubber composition for tires can be manufactured by kneading and mixing the above components using a conventional rubber mixing machine, such as a Banbury mixer, kneader, or roll. The prepared rubber composition can be used in a conventional manner to form the tread portion of a stud tire and then vulcanized.

[0043] The tire rubber composition of the present invention, configured as described above, preferably has a glass transition temperature of -60°C or lower, more preferably -100°C to -65°C. Having such a glass transition temperature is advantageous for improving ice performance. If the glass transition temperature of the tire rubber composition exceeds -60°C, ice performance deteriorates. The glass transition temperature is determined by measuring a thermogram using differential scanning calorimetry (DSC) at a heating rate of 20°C / min, and taking the temperature at the midpoint of the transition region.

[0044] As described above, the rubber composition for tires of the present invention can achieve a high degree of both excellent ice performance and wear resistance. Therefore, it can be suitably used in the tread portion of studless tires. Studless tires in which the tread portion is molded with the rubber composition for studless tires of the present invention can effectively exhibit ice performance and wear resistance due to the above-described physical properties of the rubber composition for studless tires.

[0045] The present invention will be further described below with reference to examples, but the scope of the present invention is not limited to these examples. [Examples]

[0046] Sixteen tire rubber compositions (Standard Example 1, Examples 1-11, Comparative Examples 1-4) with the compositions listed in Tables 1 and 2 were prepared. The components excluding sulfur, vulcanization accelerator, and particles were kneaded in a 1.7 L Banbury mixer for 5 minutes, and released when the temperature reached 145°C to obtain a masterbatch. Sulfur, vulcanization accelerator, and particles were added to the obtained masterbatch and kneaded in an open roll at 70°C to obtain sixteen tire rubber compositions.

[0047] The obtained tire rubber composition was vulcanized at 170°C for 10 minutes using a mold of a predetermined shape (internal dimensions: length 150 mm, width 150 mm, thickness 2 mm) to prepare vulcanized rubber test specimens. Tables 1 and 2 show the average glass transition temperature (average Tg) and hardness measured using the obtained vulcanized rubber test specimens. The average glass transition temperature was determined for each vulcanized rubber test specimen by measuring a thermogram using differential scanning calorimetry (DSC) at a heating rate of 20°C / min, and the value was obtained as the midpoint temperature of the transition zone. The hardness was measured in accordance with JIS K6253.

[0048] Using the obtained vulcanized rubber test pieces, the ice performance and abrasion resistance were evaluated using the test methods described below.

[0049] Ice performance The obtained vulcanized rubber test specimens were attached to a flattened cylindrical rubber base, and tested using an inside drum type ice friction tester at a temperature of -1.5°C and a load of 5.5 kg / cm². 2 The coefficient of friction on ice was measured under the condition of a drum rotation speed of 25 km / h. The obtained coefficient of friction on ice was converted into an index with the standard example value set to 100, and is shown in the "Ice Performance" column. A larger index value indicates a larger coefficient of friction on ice and superior ice performance.

[0050] Wear resistance The obtained vulcanized rubber test pieces were subjected to abrasion testing using a Lambourn abrasion tester (manufactured by Iwamoto Seisakusho Co., Ltd.) in accordance with JIS K6264, under the conditions of a temperature of 20°C, a load of 39N, a slip ratio of 30%, and a time of 4 minutes. The obtained results are expressed as an index with the reciprocal of the standard example set to 100, and are shown in the "Abrasion Resistance" column. A higher index indicates better abrasion resistance.

[0051] [Table 1]

[0052] [Table 2]

[0053] The types of raw materials used in Tables 1 and 2 are shown below. • NR: Natural rubber, TSR20 • BR: Butadiene rubber, manufactured by Nippon Zeon Corporation, Nipol BR1220 • Modified BR: Modified butadiene rubber, manufactured by Nippon Zeon Corporation, Nipol BR1250H • CB: Carbon Black, manufactured by Cabot Japan, Show Black N339 • White filler: Silica, manufactured by Rhodey Corporation, Zeosil 1165MP (CTAB specific surface area: 159 m²) 2 / g) • Particle 1: Particles obtained by the preparation method described later. • Particle 2: Particles obtained by the preparation method described later. • Particle 3: Particles obtained by the preparation method described later. • Particle 4: Particles obtained by the preparation method described later. • Silane coupling agent: Si69 manufactured by Evonik Degussa. • Oil: Showa Shell Sekiyu Extract No. 4 S • Liquid polymer 1: Liquid polybutadiene, manufactured by Kuraray Co., Ltd., LBR302 (weight-average molecular weight: 5500) • Liquid polymer 2: Liquid SBR, manufactured by Kuraray Co., Ltd., LSBR841 (weight-average molecular weight: 10000) • Orange oil: Aromatic modified terpene resin, manufactured by Yasuhara Chemical Co., Ltd. (YS Resin TO125) • Anti-aging agent: SANTOFLEX 6PPD manufactured by Solutia Europe • Wax: Paraffin wax manufactured by Ouchi Shinko Chemical Industry Co., Ltd. • Sulfur: Finely powdered sulfur containing Kinka oil, manufactured by Tsurumi Chemical Industry Co., Ltd. • Vulcanization accelerator: Noxellar CZ-G, manufactured by Ouchi Shinko Chemical Industry Co., Ltd.

[0054] Method for preparing particle 1 An aqueous dispersion medium was prepared by mixing 200 parts by mass of ion-exchanged water with 50 parts by mass of sodium chloride, 20 parts by mass of colloidal silica with a silica active ingredient content of 20% by mass, and 0.5 parts by mass of an aqueous solution of adipic acid-diethanolamine condensate with an effective concentration of 50%, and then adjusting the pH to 2.8-3.2. Separately, 95 parts by mass of n-butyl acrylate, 3 parts by mass of ethylene glycol dimethacrylate, 1 part by mass of PEG#400 diacrylate, 1 part by mass of polybutadiene diacrylate with a weight-average molecular weight of 10000, and 3 parts by mass of di-2-ethylhexyl peroxydicarbonate with an active ingredient content of 70%, and dissolving them to prepare an oily mixture. The aqueous dispersion medium and the oily mixture obtained above were stirred (8000 rpm x 1 min) in a TK homomixer type 2.5 (Primix Corporation) to prepare a suspension. This suspension was transferred to a 1.5-liter pressurized reactor, purged with nitrogen, and the initial reaction pressure was set to 0.2 MPa. Polymerization was carried out at a polymerization temperature of 65°C for 15 hours while stirring at 80 rpm to obtain an aqueous dispersion containing particles. The obtained aqueous dispersion containing particles was filtered and dried to obtain particle 1. The obtained particle 1 was a perfectly spherical particle with an average particle size of 25 μm and a glass transition temperature of -39°C.

[0055] Method for preparing particle 2 Particle 2 was obtained by adjusting the preparation in the same manner as Particle 1, except that 30 parts by mass of colloidal silica containing 20% ​​by mass of silica as the active ingredient was replaced with 91 parts by mass of n-butyl acrylate and 5 parts by mass of polybutadiene diacrylate with a weight-average molecular weight of 10,000. The obtained Particle 2 was a perfectly spherical particle with an average particle size of 12 μm and a glass transition temperature of -35°C.

[0056] Method for preparing particle 3 Particle 3 was obtained by preparing the mixture in the same manner as Particle 1, except that 96 parts by mass of n-butyl acrylate and 0 parts by mass of polybutadiene diacrylate with a weight-average molecular weight of 10,000 were replaced. The obtained Particle 3 was a perfectly spherical particle with an average particle size of 28 μm and a glass transition temperature of -40°C.

[0057] Method for preparing particle 4 Particle 4 was obtained by preparing the mixture in the same manner as Particle 1, except that 96 parts by mass of n-butyl acrylate and 0 parts by mass of PEG#400 diacrylate were replaced. The obtained particle 4 was a perfectly spherical particle with an average particle size of 35 μm and a glass transition temperature of -41°C.

[0058] The glass transition temperature of the obtained particles was determined by measuring a thermogram using differential scanning calorimetry (DSC) at a heating rate of 20°C / min, and the temperature at the midpoint of the transition region was taken. The average particle size of the obtained particles was the average value of the particle sizes of at least 100 particles measured from microscope images taken at 1000x to 5000x magnification.

[0059] As is clear from Tables 1 and 2, the tire rubber compositions of Examples 1 to 11 exhibited better ice performance than Standard Example 1, while also demonstrating excellent abrasion resistance equivalent to or better than Standard Example 1, which does not contain particles, thus achieving a high degree of balance between these performances. On the other hand, Comparative Examples 1 and 2 used particles 3 that did not contain monomer (B), i.e., diene monomers, resulting in poor abrasion resistance. Comparative Example 3 had poor abrasion resistance due to a low amount of white filler. Comparative Example 4 had poor abrasion resistance due to an excessive amount of particles.

Claims

1. A rubber composition for studless tires comprising 100 parts by mass of diene rubber, 30 parts by mass or more of a white filler, and 0.5 to 30 parts by mass of particles having a glass transition temperature of -70°C to 0°C, The particles consist of a polymer of a monomer mixture containing monomer (A), monomer (B), and monomer (C). The monomer (A) is a monofunctional (meth)acrylic acid ester monomer, The monomer (B) has at least two (meth)acryloyl groups, a weight-average molecular weight of 500 to 50000, and is a diene monomer represented by the following general formula (1). R 1 -O-R 2 -O-R 3 (1) (In the formula, R1 and R3 are (meth)acryloyl groups, and R2 is a structure that includes a polymer chain having a diene as a constituent unit.) The monomer (C) is a polyfunctional vinyl monomer other than the monomer (B), A rubber composition for studless tires, characterized in that the mass ratio of monomer (A) in the monomer mixture is 80% to 99.5% by mass, the mass ratio of monomer (B) in the monomer mixture is 0.1% to 5% by mass, and the mass ratio of monomer (C) in the monomer mixture is 0.4% to 15% by mass.

2. The rubber composition for studless tires according to claim 1, characterized in that the diene rubber comprises polybutadiene rubber and natural rubber, and the content of natural rubber is 30% by mass or more with respect to 100% by mass of the polybutadiene rubber.

3. The rubber composition for studless tires according to claim 1 or 2, characterized in that the average particle diameter of the aforementioned particles is 3 μm to 70 μm.

4. A rubber composition for studless tires according to any one of claims 1 to 3, characterized in that the glass transition temperature is -60°C or lower.

5. A rubber composition for studless tires according to any one of claims 1 to 4, further comprising a liquid polymer having a weight-average molecular weight of 1,000 to 100,000.

6. The rubber composition for studless tires according to any one of claims 1 to 5, characterized in that the diene rubber further comprises a terminal-modified diene rubber.

7. A rubber composition for studless tires according to any one of claims 1 to 6, further comprising an aromatically modified terpene resin.

8. A studless tire characterized by having a tread portion molded with the rubber composition for studless tires described in any one of claims 1 to 7.

Images